This invention relates generally to propeller blades for airplanes, and more particularly to a fiber-reinforced resin composite blade attached to a unitary metal ferrule and a method of manufacture.
Modern propellers for small airplanes and airboats typically consist of an assembly of two, three, or more blades attached symmetrically around a rotatable hub. The blade is often machined from aluminum, or may be fabricated from fiber-reinforced resin, such as graphite fibers embedded in an epoxy matrix.
Aluminum blades are generally strong. A disadvantage is that the edges of an aluminum blade usually get heavily nicked and pitted by gravel and other objects, requiring remachining of the edges. Another disadvantage is that metal blades are heavy, compared to synthetic composite blades. All materials for airplanes are preferably light, but it is especially desirable for the propeller to be light, so that the center of gravity of the airplane is not near the nose of the airplane. Another disadvantage is the cost of the material and the cost of machining the blade.
Resin/fiber composite blades are lighter than aluminum blades. An advantage of composite materials is that the fibers can be selected and oriented to yield a blade with more stiffness where needed and more ductility where needed. For example, glass fiber is more ductile than graphite; graphite fiber is stiffer than glass and has greater tensile strength. So that articles of consistent quality and strength can be fabricated from resin/fiber composites, fabricators often use xe2x80x9cpre-preg,xe2x80x9d that is, a fibrous material pre-impregnated with resin. Typically, the fibrous material is saturated with liquid resin and heated very gently to cause the resin to gel to what is called the xe2x80x9cB-stage,xe2x80x9d but not to cure.
B-stage resin contains very little solvent and the molecules of polymer are close together but not cross-linked. The resin cannot flow at room temperature, but generally does soften when heated. B-stage pre-preg material up to several thousandths of an inch in thickness can be cut with scissors and feels like stiff paper or manila stock.
Other methods are also used, such as xe2x80x9cwet lay-up,xe2x80x9d wherein the fibrous material is saturated with liquid resin by the fabricator and laid up into the mold, without and intermediate B-stage.
The orientation of the fibers within the resin matrix has a large affect on the stiffness of the resulting resin/fiber composite. Fibers may be woven or knit before impregnation, or may be aligned parallel to each other. When the fibers are aligned parallel, the resulting pre-preg is called xe2x80x9cunidirectional.xe2x80x9d Unidirectional resin/fiber composite is flexible if bent parallel to the fibers and stiff if bent across the fibers. Woven and knit textiles also have characteristic flexing properties.
A relatively well-known method of fabricating propeller blades, called xe2x80x9ccompression molding,xe2x80x9d starts with machining a mold having two halves, each having a cavity the shape of one side of the blade. Uncured fiber/resin composite material is arranged in the cavities according to a design plan called a xe2x80x9clay-up schedule.xe2x80x9d The lay-up schedule specifies the shape and fiber orientation of the pieces, which are overlapped to yield the desired tapering shape. Splitting the mold into two halves is generally preferred for molding an article having bilateral symmetry; an article with a higher degree of symmetry might preferably be molded from a mold divided into a higher number of segments.
Typically, uncured resin/fiber composite is laid up in each mold half and an insert of wood or polyurethane foam is placed in one of the halves. Strengthening inserts, typically machined from titanium or aluminum, are also generally included. The halves of the mold are brought together and clamped. This xe2x80x9cmold assembly,xe2x80x9d consisting of metal mold halves and inserts, is heated to the curing temperature, such as by being placed in an oven.
During curing, the resin flows to join the uncured resin/fiber composite into a fairly uniform mass, which adheres to the insert. The resin eventually crosslinks and becomes rigid. After the resin is cured, it will not soften again upon heating.
One disadvantage of using compression molding to make propeller blades is the high cost. Both materials and labor for compression molded propeller blades are expensive. For example, U.S. Pat. No. 4,810,167 of Spoltman et al. discloses a propeller blade that contains multiple machined components. Some of the components are laid up in the mold assembly; others are glued on after molding. The base of the molded blade must be precision machined to accept these glued-on components.
This method also has a large indirect cost, which is inflexibility of mold use. Because there are critical mating surfaces on the foam and metal inserts, the post-assembled components (components attached to the molded article after cure), as well as the mold cavities, each mold is dedicated to making a single design of blade. Even the ability to change the lay-up schedule to make a blade of the same shape, but different stiffness characteristics, is seriously limited. Because molds are typically the most expensive and longest lead-time part of new design, it is undesirable to have to make a new mold to accommodate every small change.
To avoid these high costs of machined parts, complicated lay-up, and single-design molds, the method known as xe2x80x9cinternal pressure molding,xe2x80x9d or xe2x80x9cIPM,xe2x80x9d has been used to make propeller blades. IPM uses a mold cavity to define the outer shape of the article being molded, as does compression molding, but an inflatable, stretchable bladder compresses the resin/fiber composite from the inside. Blades made using IPM are hollow; no foam or wood insert is needed. Eliminating the insert decreases machining cost, assembly cost, and weight of the finished article. The bladder conforms to the profile of the composite material, thus one mold can accommodate nearly any lay-up schedule that yields the same external shape. New lay-up schedules can be tested very cheaply and fewer molds are needed in the shop.
A major disadvantage of conventional IPM composite blades is that they are not as strong as solid aluminum or compression molded blades having metal inserts and glued-on reinforcing components. The root of the blade, especially the portion that attaches to the hub, must withstand high dynamic force and fatigue. The root of the blade can be reinforced by attaching two halves of a split collar around it after it is molded. This strengthens the blade, but neutralizes some of the cost savings of IPM. A split collar is also not as strong as a unitary collar or ferrule is. As a result, the market for IPM airplane blades has been limited to relatively low-performance craft. There is a need for a simple, inexpensive IPM airplane propeller blade that is strong enough to be used on the most demanding airplanes, such as competitive aerobatic planes.
This invention is an IPM composite blade assembly that is suitable for competitive aerobatic airplanes, or for other uses, such as airboats. The blade assembly includes a fiber-reinforced resin blade, means for mounting the assembly onto a propeller hub, and a unitary ferrule for reinforcing the root of the blade.
The unitary ferrule is co-molded with the blade and preferably forms a part of the mold. Assembly of the ferrule into the lay-up is very simple and quick. Because the unitary ferrule does not have to be split to go over the molded root of the blade, it is very strong.
The uncured composite material is laid up into a standard segmented mold. An inflatable bladder is laid between the halves of the mold before the mold is closed. Each cavity segment includes a recess segment that accommodates half of the ferrule and the end of the recess segment is open at the end. The uncured composite material laid into the ferrule recess forms the root of the blade.
The mold segments are mated together with the cavity segments facing each other. The laid-up uncured composite material and the bladder are within the mold cavity, which consists of the two cavity segments. After the segments of the mold are mated, i.e., the mold is xe2x80x9cclosedxe2x80x9d, the unitary ferrule is slipped into the open end of the mold, into the recess comprising the recess segments.
The combination of closed mold, bladder, and ferrule are herein called the xe2x80x9cmold assembly.xe2x80x9d When the mold assembly is heated, the resin matrix of the composite material flows, as described above. The resin wets the inner bore of the unitary ferrule and forms a strong adhesive bond to it. This bond is stronger than the one formed by gluing metal components onto the cured composite, because the bond extends deep within the matrix of the root portion.
The unitary ferrule preferably includes mechanical means for strengthening the bond, such as deep grooves and ridges on the inner bore. The main force on the blade during flight is parallel to the longitudinal axis of the blade (centripetal force from rotation of the propeller assembly). The grooves and ridges cause much of the root portion of the blade to be under compression, strengthening the adhesive bond between the root portion and the inner bore, as well as locking the blade mechanically to the ferrule.
The blade assembly includes mounting means for mounting the blade assembly onto a standard type of propeller hub. The mounting means includes the ferrule. For example, the ferrule may include a circular flange around its perimeter that mates with attachment means in the hub. The mounting means typically includes critically-dimensioned surfaces. Because the outer surface of the ferrule is already precision machined to fit the cavity of the mold, using the outer surface of the ferrule as a mounting means does not add cost to the ferrule or to the blade assembly. The inner bore does not generally require precision machining. The low number of machined components and lower number of critical surfaces helps prevent machine tools from xe2x80x9cbottleneckingxe2x80x9d the manufacturing process.
Assembly of this reinforced blade is scarcely any more difficult than assembly of an un-reinforced IPM blade. Due to the conformability of the bladder, a single mold is used for all blade assemblies that have the same outside shape. Blade assemblies having different lay-up schedules and different conformation of the inner bore of the ferrule are molded on the same molds. This increases productivity by allowing a mold to be used for several different products, minimizing the mold""s idle time. Molds can be made even more flexible by using removable inserts to provide the recesses for different ferrules. Such an insert would be handled only once during set-up for a given product run and again during break-down; hence would not increase the handling required for each blade assembly.
The blade assembly and method of the present invention allows propeller assemblies to be made that have strength and performance comparable to propellers made by compression molding or by machining of metal, but with less weight and a fraction of the cost. Propeller blades assemblies made according to the present invention are comparable in cost to non-reinforced blades made by IPM and blades reinforced with split collars, but are stronger and longer-lasting.